This disclosure concerns an invention relating generally to combustion methodologies for internal combustion engines, and more specifically to combustion methodologies resulting in decreased pollutant emissions.
For better understanding of the invention described in this document, it is initially useful to review basic principles of internal combustion engine structure and operation. FIG. 1 depicts a cylinder in a simple idealized internal combustion engine 100, including a combustion chamber 102 defined between a piston 104 and a cylinder head 106. The cylinder head 106 includes a fuel injector 108 and a pair of combustion chamber valves, an intake valve 110 for intake of air from an intake manifold 112, and an exhaust valve 114 for exhaust of combustion products to an exhaust manifold 116 and exhaust system. As is well known, the engine 100 operates by engaging in a combustion cycle, wherein fuel is burned in the combustion chamber 102 to expand the gases (primarily air) therein and drive the piston 104. The piston 104 in turn drives a crank 118 associated with the piston 104, with the crank 118 in turn driving the crankshaft (not shown) which provides power output for a vehicle drive train or to other structures for transmitting mechanical power. The classical four-stroke combustion cycle for both SI (spark ignition or gasoline) engines and CI (compression ignition or diesel) engines involves the following steps:
(1) An intake stroke, wherein the intake valve 110 is opened while piston 104 retreats from cylinder head 106 to draw air into the combustion chamber 102 from the intake manifold 112.
(2) A compression stroke, wherein the piston 104 approaches cylinder head 106 with the combustion chamber valves 110 and 114 closed (at least during the latter portion of the stroke).
(3) A power or expansion stroke, wherein fuel injected into the combustion chamber 102 is ignited and the expanding gases within the combustion chamber 102 push the piston 104 outwardly (as during the intake stroke). Again, the combustion chamber valves 110 and 114 usually remain closed (at least during the early portion of the stroke).
(4) An exhaust stroke, wherein the combustion products within the combustion chamber 102 are expelled to the exhaust manifold 116 by advancing the piston 104 towards the cylinder head 106 with the exhaust valve 114 open.
Each stroke occurs over 180 degrees of crankshaft rotation, with the entire cycle thereby occurring over 720 degrees (two full crankshaft revolutions). The combustion chamber valves 110 and 114 are usually opened and closed at the desired times by valve actuators such as cams or other structures, which are in turn driven by the crankshaft (not shown). Since such arrangements couple the timing and extent of valve opening and closing to the positioning of the crankshaft, and since it may be desirable to have a greater degree of control over valve actuation to achieve desired combustion chamber conditions, there has been a recent trend towards the use of variable valve timing technologies. These technologies wholly or partially decouple the timing and/or extent of valve actuation from the crankshaft position, and allow the valves 110 and 114 to be opened and/or closed when desired (and may also allow the degree of opening to be varied as desired). Examples of variable valve actuation (VVA) schemes may be found, for example, in U.S. Pat. Nos. 4,777,915; 4,829,947; and 5,515,818.
The foregoing combustion cycle steps/strokes differ between classical SI and CI engines in that SI engines tend to inject fuel during the intake stroke, whereas CI engines tend to inject fuel late in the compression stroke or early in the power stroke, close to xe2x80x9ctop dead centerxe2x80x9d (TDC), the piston 104""s point of closest approach to the cylinder head 106. Additionally, in SI engines, ignition of the fuel/air mixture occurs by introduction of a spark (with no spark plug being illustrated in FIG. 1). In contrast, classical CI engines rely on the compression stroke to increase the heat and pressure in the combustion chamber 102 to such a degree that ignition results. There are also various types of xe2x80x9chybridxe2x80x9d engines which operate using a combination of SI and CI principles, or example, engines which run primarily on CI principles but which use a spark or xe2x80x9cglow plugxe2x80x9d to assist with ignition. (Also note that the engine of FIG. 1 is described as a xe2x80x9csimple idealizedxe2x80x9d one since real-world engines may have a wide variety of combustion chamber configurations other than those shown at 102 in FIG. 1, and may have varying numbers, locations, and configurations of combustion chamber valves 110 and 114 and/or injectors 108.)
In the field of engine development and manufacture, two concerns of critical importance are engine efficiency (e.g., power output per fuel consumption) and engine emissions. Diesel engines tend to operate more efficiently than SI engines, but they unfortunately also tend to have much greater pollutant emissions than SI engines. Common pollutants arising from the use of internal combustion engines are nitrogen oxides (commonly denoted NOx) and particulates (also known simply as xe2x80x9csootxe2x80x9d). NOx is generally associated with high-temperature engine conditions, and may be reduced by use of measures such as exhaust gas recirculation (EGR), wherein the engine intake air is diluted with relatively inert exhaust gas (generally after cooling the exhaust gas). This reduces the oxygen in the combustion chamber and obtains a reduction in maximum combustion temperature, thereby deterring NOx formation. Particulates (soot) include a variety of matter such as elemental carbon, heavy hydrocarbons, hydrated sulfuric acid, and other large molecules, and are generally associated with incomplete combustion. Particulates can be reduced by increasing combustion and/or exhaust temperatures, or by providing more oxygen to promote oxidation of the soot particles. Unfortunately, measures which reduce NOx tend to increase particulate emissions, and measures which reduce particulates tend to increase NOx emissions, resulting in what is often termed the xe2x80x9csoot-NOx tradeoffxe2x80x9d.
At the time of this writing, the diesel engine industry is facing stringent emissions legislation in the United States, and is struggling to find methods to meet government-imposed NOx and soot targets for the years 2002-2004 and even more strict standards to be phased in starting in 2007. One measure under consideration is use of exhaust after-treatment (e.g., particulate traps) for soot emissions control in both heavy-duty truck and automotive diesel engines. However, in order to meet mandated durability standards (e.g., 50,000 to 100,000 miles), the soot trap must be periodically regenerated (the trapped soot must be periodically re-burned). This requires considerable expense and complexity, since typically additional fuel must be mixed and ignited in the exhaust stream in order to oxidize the accumulated particulate deposits.
Apart from studies directed to after-treatment, there has also been intense interest in the more fundamental issue of how to reduce NOx and particulates generation from the combustion process and thereby obtain cleaner xe2x80x9cengine outxe2x80x9d emissions (i.e., emissions directly exiting the engine, prior to exhaust after-treatment or similar measures). Studies in this area relate to shaping combustion chambers, timing the fuel injection, tailoring the injection rate during injection so as to meet desired emissions standards, or modifying the mode of injection (e.g, modifying the injection spray pattern). One field of study relates to premixing methodologies, wherein the object is to attain more complete mixing of fuel and air in order to simultaneously reduce soot and NOx emissions. In diesel engines, the object of premixing methodologies is to move away from the diffusion burning mechanism which drives diesel combustion, and instead attempt to attain premixed burning. In diffusion burning, the oxidant (fuel) is provided to the oxidizer (air) with mixing and combustion occurring simultaneously. The fuel droplets within an injected spray plume have an outer reaction zone surrounding a fuel core which diminishes in size as it is consumed, and high soot production occurs at the high-temperature, fuel-rich spray core. In contrast, premixed burning mixes fuel and air prior to burning, and the more thorough mixing results in less soot production. Premixing may be performed by a number of different measures, such as by use of fumigation (injection of fuel into the intake airstream prior to its entry into the engine), and/or direct injection of a fuel charge relatively far before top dead center (TDC) so that motion of the piston 104, and convection within the cylinder, result in greater mixing.
One promising diesel premixing technology is HCCI (Homogeneous Charge Compression Ignition), which has the objective of causing initial ignition of a lean, highly premixed air-fuel mixture at or near top dead center (TDC). An extensive discussion on HCCI and similar premixing techniques is provided in U.S. Pat. No. 6,230,683 to zur Loye et al., and U.S. Pat. No. 5,832,880 to Dickey and U.S. Pat. No. 6,213,086 to Chmela et al. also contain useful background information. The charge is said to be xe2x80x9chomogeneousxe2x80x9d in HCCI because it is (at least theoretically) highly and evenly mixed with the air in the cylinder. Ignition is then initiated by autoignition, i.e., thermodynamic ignition via compression heating. The objective of HCCI is to use autoignition of the lean and homogeneous mix to provide a uniform and relatively slow non-diffusion (or minimized diffusion) burn, resulting in significantly lower combustion chamber temperatures and diminished NOx production (which thrives at high temperature), as well as lower soot production owing to enhanced mixing. In contrast, a richer mixture (such as that necessary for flame propagation from the spark in an SI engine) will burn more quickly at greater temperature, and therefore may result in greater NOx production.
Another promising premixing technology is the Modulated Kinetics (MK) technique, which might be regarded as being a species of HCCI. MK combustion is primarily characterized by three features: (1) a leaner-than-usual fuel/air mixture is used; (2) injection is made at or near top dead center (often after TDC); and (3) the ignition delay exceeds the injection duration (so that the fuel/air mixture is at least partially premixed prior to combustion). In essence, MK combustion involves use of relatively late injection(s) in the xe2x80x9cgeneralxe2x80x9d HCCI combustion method described above, and concentrates on using the ignition delay for premixing. For further discussion on MK combustion, see, e.g., Kimura et al., xe2x80x9cUltra-clean combustion technology combining a low-temperature and premixed combustion concept for meeting future emissions standards,xe2x80x9d SAE paper 2001-01-200, 2001.
As the foregoing references note, while HCCI-type premixing processes might be beneficially implemented in CI engines to achieve their high efficiency without their customarily high emissions, HCCI is also hard to accomplish owing to the difficulties in igniting the lean mix and/or controlling the start of ignition. Combustion in an SI engine is readily initiated by the spark, with premixed burning occurring afterward; similarly, combustion in a conventional CI engine is initiated by fuel injection near top dead center (at or slightly after the end of the compression stroke) when thermodynamic conditions for autoignition are favorable, with diffusion burning occurring afterward. However, HCCI does not utilize a spark, nor is it desirable for HCCI to use the rich mixture needed for effective use of a spark. It is also difficult for HCCI to achieve a homogeneous charge or premixed burning if injection near top dead center is used, since there is less time for mixing to occur before ignition. Thus, a key area of study in the HCCI field is how to efficiently initiate ignition, and more critically, how to effect ignition at the desired time. While these issues are somewhat straightforward where CI engines operate under a relatively rigid set of operating conditions (as with diesel generators, which tend to operate at relatively constant speeds and loads), these issues become exceedingly complex where CI engines must operate at varying speeds and loads (as in automotive/vehicular operations). These ignition and timing problems are the primary reason why HCCI methodologies have not attained widespread use outside of generators and other constant speed/load applications.
In view of the foregoing discussion, there is a significant need for methods and apparata which assist in obtaining premixed burning, particularly premixed burning which achieves or approximates HCCI burning, so as to allow exploitation of the high efficiency of CI engines without the detriment of their high pollutant emissions.